Phase change memory device comprising bismuth-tellurium nanowires

ABSTRACT

The present invention relates to a phase change memory device comprising bismuth-tellurium nanowires. More specifically, the bismuth-tellurium nanowires having PRAM characteristics may be prepared by using a porous nano template without any high temperature process and said nanowires may be used in the phase change memory device by using their phase change characteristics to identify memory characteristics.

This application claims the benefit of Korean Patent Application No.10-2010-0078800, filed on Aug. 16, 2010, which is hereby incorporated byreference for all purposes as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to a phase change memory device comprisingbismuth-tellurium nanowires, and more specifically, a phase changememory device comprising bismuth-tellurium nanowires which may be usedin the phase change memory device by preparing bismuth-telluriumnanowires having a PRAM (Phase Change Random Access Memory)characteristic using a porous nano template without any high temperatureprocess, and indentifying memory switching by means of the phase changecharacteristic of said nanowires.

BACKGROUND OF THE INVENTION

Binary information is recorded, deleted and deciphered in memories usingchange of physical properties in electrical, magnetic, opticalcharacteristics, and the like, of materials. Among such memories, a PRAM(Phase change Random Access Memory) to be in the limelight as theupcoming nonvolatile memory is a memory that information is recorded,deleted and deciphered using change in optical and electricalcharacteristics of materials according to the phase change. An opticalmemory utilizes reflectance to be different on changing a material statewith a laser, while a phase change memory utilizes change of electricalconductance (specific resistance) on changing it with electricalsignals. That is, when a material is in an amorphous phase, it has veryhigh specific resistance. When it is in a crystalline phase that atomsare regularly arranged, it has low specific resistance. Then, binaryinformation may be stored, deleted and deciphered using a differentelectrical conductance according to such a state.

When upper and bottom metal electrodes are linked to a power circuit,following applying voltage, current is carried through a part that aphase change material is in contact with the electrodes, wherein Joule'sheat is generated at the contact areas according to Joule's law. As theelectrical conductance (specific resistance) is changed on changing thematerial state by Joule's heat, a crystalline phase material and anamorphous material may be switched into an amorphous phase material anda crystalline phase material, respectively, using electrical signals.Since the resulting two phases are very stable, they are not erased eventhough power of the device is off, so that they may be used asnonvolatile memories.

In other words, recording of information (SET) refers to changing thematerial state from an amorphous phase (off state) to a crystallinephase (on state), for which long low current pulses are provided for anamorphous phase material (FIG. 1 a). On the contrary, if short highcurrent pulses are provided for a crystalline material being in on stateto delete information (RESET), the crystalline phase is changed into theamorphous phase to be in off state (FIG. 1 b).

Such a phase change memory has a simple structure compared with othernonvolatile memories and a benefit that an input-output speed is fast.It has been developed as a device having a thin film form based on GST(Ge, Sb, Te) being chalcogenides so far, and also researched formaterials by means of doping elements such as Se and Bi. Recently, avariety of trials are carried out by realizing a PRAM or forming a thinfilm with nanowires prepared by high temperature processes such as CVD.However, in such processes, there are disadvantages that compositionratios in said elements are not easily controlled and they havecomplicated preparation processes and high production costs per unit.

BRIEF SUMMARY OF THE INVENTION

The present invention is devised to improve the prior art problems andintended to provide a phase change memory device prepared byelectrolytically depositing bismuth-tellurium nanowires having a phasechange characteristic in a porous nano template without any hightemperature process, and a method of preparing the same.

To achieve said object, the present invention provides a phase changememory device comprising a substrate; and at least one Bi_(x)Te_(y)(x/y<1) nanowire formed on said substrate.

The present invention also provides a method of preparing the presentphase change memory device comprising steps of

-   -   immersing a template formed on a substrate and having at least        one pore in an electrolyte containing a bismuth precursor and a        tellurium precursor; and    -   electrolytically depositing Bi_(x)Te_(y) (x/y<1) nanowires in        the pores of said template by applied voltage.

The present invention also provides a method of deleting information inthe present phase change memory device comprising a step of applyingvoltage pulses of 7 to 8 V and 30 to 50 ns to at least one Bi_(x)Te_(y)(x/y<1) nanowire formed on a substrate.

The present invention also provides a method of recording information inthe present phase change memory device comprising a step of applyingvoltage pulses of 1 to 2 V and 90 to 110 ns to at least one Bi_(x)Te_(y)(x/y<1) nanowire formed on a substrate.

The present invention also provides an electronic device comprising aphase change memory device according to the present invention.

In addition to benefits that bismuth-tellurium is electrolyticallydeposited in a porous template to prepare nanowires having a PRAMcharacteristic without any high temperature process, easy and promptproduction may be possible due to no high temperature process and theunit cost of production may decrease, the present invention hasadvantages that low programming current may be expected, since nanowireshave low cell volume for the melting point to be reduced and a thermalinterference which can be generated on being scaled down, may beinhibited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents characteristics of pulses applied for forming acrystalline condition and an amorphous condition in a phase changememory.

FIG. 2 depicts a schematic view of procedures for preparing a phasechange memory device according to the present invention.

FIG. 3 depicts a schematic view of procedures for an anodizationexperiment.

FIG. 4 is top and side electron micrographs of an anodic aluminum oxide(AAO) template in a phase change memory device according to the presentinvention.

FIG. 5 is top and side electron micrographs of a Bi₂Te₃ nanowire arrayaccording to the present invention.

FIG. 6 depicts a schematic view of an electrolytic polishingexperimental apparatus for triode electrochemical deposition accordingto one aspect of the present invention.

FIG. 7 represents an X-ray diffraction pattern of the Bi₂Te₃ nanowireaccording to the present invention.

FIG. 8 is transmission electron micrographs of the Bi₂Te₃ nanowiresfully separated from the AAO template according to the presentinvention, wherein (a) represents check and diffraction patterns of theBi₂Te₃ nanowires in a crystalline phase and (b) represents the amorphousphase boundary of the Bi₂Te₃ nanowires by phase change.

FIG. 9 represents resistance change characteristics of the Bi₂Te₃nanowire device of the present invention depending on temperature,wherein direction of the arrows represents that of resistance changedepending on temperature change.

FIG. 10 represents resistance change characteristics of the Bi₂Te₃nanowire device of the present invention, wherein (a) is FESEM images ofindividual nanowire devices, (b) represents resistance changecharacteristics of the nanowire in the initial amorphous phase dependingon the applied voltage, and (c)) represents resistance changecharacteristics of the Bi₂Te₃ nanowire device in a conditionphase-changed by applying voltage thereto.

FIG. 11 represents resistance change characteristics of the Bi₂Te₃nanowire array device of the present invention depending on the repeatedelectric pulses.

FIG. 12 represents phase change characteristics of the Bi₂Te₃ nanowiresof the present invention depending a composition ratio.

FIG. 13 depicts a schematic view of a phase change memory devicedeposited with the bismuth-tellurium nanowires of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The constitution of the present invention is specifically explainedbelow.

The present invention relates to a phase change memory device comprisinga substrate; and at least one Bi_(x)Te_(y) (x/y<1) nanowire formed onsaid substrate.

The present phase change memory device may comprise one pair ofelectrodes to which voltage is applied. Preferably, said electrode maybe a metal electrode.

In addition, said nanowire may be formed in pores of a templatepositioned on a substrate.

Said template may have at least one pore. Preferably, it may be anodicaluminum oxide (AAO).

Said Bi_(x)Te_(y) (x/y<1) nanowire, wherein x is 2 and y is 3, ispreferred.

Said nanowire is in an amorphous phaseon applying voltage pulses (AP) of7 to 8 V and 30 to 50 ns and changed to a crystalline phaseon applyingvoltage pulses (CP) of 1 to 2 V and 90 to 110 ns, so that it ischaracterized by having a PRAM (phase change random access memory)characteristic which represents phase change depending on the appliedvoltage pulse amplitude and time.

According to one embodiment of the present invention, said nanowireshows a tendency that its resistance value is slightly reduced, onincreasing temperature, because the nanowire is in a crystalline phase.The resistance value rapidly increases at about 231° C., but it remainsin high resistance condition, even if the temperature again decreases.Here, since the resistance increases at the same time as the phasechange occurs from the crystalline phase to the amorphous phase, it canbe noted that such resistance increase results from the phase change ofthe nanowire.

According to other embodiment of the present invention, the phase changememory controls the electrical conductance (specific resistance) byapplying electric pulses and generating Joule's heat at the contact areato change the material phase. If voltage pulses (AP) of 7.5 V and 40 nsare applied to said nanowire being the initial crystalline phase, thenanowire is changed to the amorphous phase having high resistance. Ifvoltage pulses (CP) of 1.5 V and 100 ns are applied thereto, it ischanged to the crystalline phase having low resistance. This change isreproducible. It is proved from such PRAM characteristics of thenanowire that said nanowire may be applied to a phase change memorydevice.

Said nanowire, a diameter of which is 10 nm to 200 nm, may be used.

In addition, said substrate may include silicon, a flexible polyimidefilm, or a polyester film, and the like, which is not particularlylimited thereto.

The present invention also relates to a method of preparing a phasechange memory device according to the present invention comprising stepsof

-   -   immersing a template formed on a substrate and having at least        one pore in an electrolyte containing a bismuth precursor and a        tellurium precursor; and    -   electrolytically depositing Bi_(x)Te_(y) (x/y<1) nanowires in        the pores of said template by applied voltage.

The method of preparing a phase change memory device of the presentinvention is characterized in that the nanowire having a PRAMcharacteristic is prepared without any high temperature process byelectrodepositing bismuth-tellurium in pores of the porous nanotemplate. There are advantages that easy and prompt production may bepossible due to no high temperature process and the unit cost ofproduction may be lowered. In addition, there are advantages that lowprogramming current may be expected, since nanowires have low cellvolume for the melting point to be reduced and a thermal interferencewhich can be generated on being scaled down, may be inhibited.

With reference to FIG. 2, the method of preparing a phase change memorydevice of the present invention is particularly explained step by stepbelow.

The first step is a step of preparing a porous template on s substrateand immersing said template in an electrolyte containing a bismuthprecursor and a tellurium precursor (FIG. 2 a-d).

Said porous template may be one having at least one pore. Morepreferably, it may be anodic aluminum oxide (AAO).

When anodic aluminum oxide (AAO) is used as said porous template,general technology may be used as procedures for forming said templateon a substrate, without being particularly limited. According to oneembodiment, it may comprise the following steps:

-   -   a first step of depositing on a substrate a metal electrode,        such Au and Pt, to be used as a bottom electrode (FIG. 2 a);    -   a second step of patterning Al on the electrode by means of a        lithography method and a lift-off method, followed by deposition        (FIG. 2 b); and    -   a third step of passivating the other part with a resist, such        that only a part to form AAO can be exposed to an electrolyte        solution, followed by anodization (FIGS. 2 c and d).

With reference to FIG. 3, said anodization method of Al is explainedbelow (FIG. 2 c, d).

Al is anodized by using an acid solution as an electrolyte foranodization and applying voltage (FIG. 3 a, b).

When the AAO layer formed after carrying out the first anodization ismelted in an acid solution such as a mixed solution of chromic acid(H₂CrO₄) and phosphoric acid (H₃PO₄), all the formed AAO layer isremoved to form a porous structure (pore) on the remaining Al layer(FIG. 3 c).

The second anodization is carried out using said porous structure as aguide, until current between the Al electrode and the carbon electrodeis to 0 (FIG. 3 d).

After anodization, an alumina (AAO) barrier is formed at the end of AAOand subsequently, removed by means of a method of immersing an AAOtemplate prepared at room temperature in an acid solution for 30 to 40minutes (FIG. 3 e).

The anodic aluminum oxide template prepared through the above steps mayhave a pore size of 50 to 200 nm and a thickness of 50 to 800 nm.

One or two or more of Bi(NO₃)₃5H₂O, BiNBO₄, Bi₂VO₅, or BiMe₂(Me₂NCH₂Ph),and the like, may be used as said bismuth precursor.

One or two or more of TeO₂, CdTe, CdZnTe, or HgCdTe, and the like, maybe used as said tellurium precursor.

Preferably, said bismuth precursor and tellurium precursor are mixedsuch that a composition ratio (Bi_(x)Te_(y)) of bismuth and tellurium isx/y<1 and more specifically, 2:3, but on departing from the above range,the nanowire shows no characteristic of phase change memory.

In the method of preparing a phase change memory device of the presentinvention, the second step is a step of applying voltage to a mixedsolution of the template and the electrolytes to electrolyticallydeposit the bismuth-tellurium nanowire in a pore of the template (FIG. 2e).

Said voltage is −0.5 to 1 V such that electrolytic deposition of thenanowire is homogeneously carried out. More specifically, the voltage ispreferably −0.1 to 0 V, wherein about −0.01 V may be used.

Triode electrochemical deposition may be used as said electrolyticdeposition, without being particularly limited thereto.

The bismuth-tellurium nanowire prepared through the above steps may be aBi_(x)Te_(y) (x/y<1) nanowire. More specifically, it may be Bi₂Te₃.

In the method of preparing a phase change memory device of the presentinvention, the third step is a step of preparing a metal electrode, forexample, Pt having a nano size, for example, 30 to 60 nm to be used as atop electrode by a DC sputter (FIG. 2 f).

In addition, the method of preparing a phase change memory device of thepresent invention may further comprise a step of immersing the templateelectrolytically deposited with a bismuth-tellurium nanowire in a basicsolution and dissolving said template.

Said basic solution may be NaOH or KOH, without being particularlylimited thereto.

The step to dissolve said template may be carried out by repeatingseveral times a process of immersing a porous template deposited withnanowires in a basic solution, centrifuging and washing a pellet withdistilled water, without being particularly limited thereto.

The present invention also relates to a method of deleting informationin a phase change memory device of the present invention comprising astep of applying voltage pulses of 7 to 8 V and 30 to 50 ns to at leastone Bi_(x)Te_(y) (x/y<1) nanowire formed on a substrate.

When voltage pulses of 7 to 8 V and 30 to 50 ns are applied to thepresent bismuth-tellurium nanowires in a phase change memory device ofthe present invention, information in the phase change memory device maybe deleted by changing the memory to an amorphous phase having highresistance.

The present invention also relates to a method of recording informationin a phase change memory device of the present invention comprising astep of applying voltage pulses of 1 to 2 V and 90 to 110 ns to at leastone Bi_(x)Te_(y) (x/y<1) nanowire formed on a substrate.

When voltage pulses of 1 to 2 V and 90 to 110 ns are applied to thepresent bismuth-tellurium nanowires in a phase change memory device ofthe present invention, information in the phase change memory device maybe recorded by changing the memory to a crystalline phase having lowresistance.

The present invention also relates to an electronic apparatus comprisinga phase change memory device according to the present invention.

In addition to benefits that bismuth-tellurium is electrolyticallydeposited in a porous template to prepare nanowires having a PRAMcharacteristic without any high temperature process, easy and promptproduction may be possible due to no high temperature process and theunit cost of production may decrease, the present invention hasadvantages that low programming current may be expected, since nanowireshave low cell volume for the melting point to be reduced and a thermalinterference which can be generated on being scaled down, may beinhibited, so that it may be applied to electronic apparatuses, such ascomputers; potable personal terminals such as PMP (Portable MultimediaPlayer), UMPC (Ultra-mobile PC), smart-phone and mini-notebook; andoptical storage information apparatuses such as CD-RW (CompactDisc-Rewritable) and DVD (Digital Video Disc).

Hereinafter, the present invention is explained in more detail viaexamples according to the present invention, but the scope of thepresent invention is not restricted to the following examples.

EXAMPLES Example 1 Preparation of Phase Change Memory Device

With reference to a schematic view of FIG. 2, a process of preparing aphase change memory device is explained below.

First, a metal electrode to be used as a bottom electrode, such as Auand Pt, was deposited on a silicon substrate (FIG. 2 a).

On the electrode, Al with a thickness of about 1 μm was patterned bymeans of a lithography method and a lift-off method, and then deposited(FIG. 2 b).

Then, the other part was passivated with a resist, such that only a partto form AAO can be exposed to an electrolyte solution, followed byanodization (FIG. 2 c, d).

With reference to FIG. 3, the method of anodizing Al is amplified below(FIG. 2 c, d).

Oxalic acid (C₂H₂O₄, 0.3 M) was used as an electrolyte for anodization,wherein the temperature was maintained at 15° C. and a voltage of 40 Vwas applied.

First, when the AAO layer formed after carrying out the firstanodization for about 2 minutes was dissolved in a mixed solution of1.8% by weight of chromic acid (H₂CrO₄) and 6% by weight of phosphoricacid (H₃PO₄) at 60° C. for about 10 minutes, all the formed AAO layerwas removed to form a porous structure (pore) on the remaining Al layer.

The second anodization was carried out using such porous structure as aguide, until current between the Al electrode and the carbon electrodewas to 0.

After anodization, an alumina (AAO) barrier was formed at the end of AAOand subsequently, removed by means of a method of immersing an AAOtemplate prepared at room temperature in 6% by weight of phosphoric acidsolution at 30° C. for 30 minutes (FIG. 3 e).

FIG. 4 is electron micrographs showing top and side appearance of theAAO template prepared via the above method.

Then, to electrolytically deposit the Bi₂Te₃ nanowire, AAO was immersedin an electrolyte that 0.01M Bi(NO₃)₃5H₂O and 0.01M TeO₂ were dissolvedin 1M HNO₃ and a voltage of −0.01 V was applied thereto (FIG. 2 e). Inthis experiment, a triode electrochemical deposition method was used(FIG. 6).

Next, Pt to be used as a top electrode was prepared as an electrodehaving about 50 nm by a DC sputter (FIG. 2 f).

Here, to remove the AAO template deposited with the Bi₂Te₃ nanowires inpores, the NaOH solution including the AAO template deposited with thenanowires was centrifuged at a speed of 4900 rpm for 15 minutes. Thesupernatant fluid was removed from the centrifuged solution, followed byfilling the remaining part with distilled water, and the resultingsolution was again centrifuged at a speed of 4900 rpm for 15 minutes,followed by removing the supernatant fluid and filling with distilledwater. Such procedures were carried out two or more times. Finally,after carrying out two or more times, the supernatant fluid was removedand filled with 99.95% ethanol.

FIG. 5 is electron micrographs showing that the nanowires areelectrolytically deposited in top and side surfaces of the AAO templateprepared via the above method.

Experimental Example 1 Investigation of Bi₂Te₃ Characteristics

FIG. 7 being a result of measuring X-ray diffraction of Bi₂Te₃ preparedin the above example 1 represented a polycrystalline structure, whereina main peak in a direction (110) and peaks in other directions arepresent.

In addition, as a result of observation for microstructures using atransmission electron microscope, the diffraction patterns (inside ofFIG. 8( a)) were identical to the results of measuring X-ray diffractionas well as the crystalline structure was observed as shown in FIG. 8(a).

The phase of Bi₂Te₃ nanowires was changed depending on temperature, andthus, for identifying change of resistance value, temperature dependencyof resistance was measured.

As shown in FIG. 9, as temperature increased, the resistance value wasslightly reduced, because the initial phase nanowires were a crystallinephase. However, the resistance value rapidly increased at about 231° C.,and the condition of high resistance was maintained, even though thetemperature was again reduced.

FIG. 8( b) is a TEM image measured after the temperature dependency ofresistance was measured.

The change of phase from a crystalline phase to an amorphous phaseoccurred, and thus, it could be seen that the increase of resistanceresults from the phase change of the Bi₂Te₃ nanowires.

FIG. 10( a) is a FESEM image of individual nanowire device. Theelectrode was prepared with platinum, and for inhibiting oxidation, aprotective film was prepared on the nanowires with silicon oxide.

In FIG. 10( b), when the nanowires were in a high resistance condition,their resistance was reduced, with changing from the amorphous phase tothe crystalline phase at about 0.5 V. Such a phase change wasreproducible as shown in FIG. 10( c).

The phase change memory controls the electrical conductance (specificresistance) by applying electric pulses and generating Joule's heat atthe contact area to change the material phase.

FIG. 11 is a characteristic of the memory measured by repeatedlyapplying voltage pulses (AP) of 7.5 V and 40 ns and voltage pulses (CP)of 1.5 V and 100 ns to a material having the initial crystalline phase.If the AP was applied thereto, the material was changed to an amorphousphase having high resistance. If the CP was applied thereto, it waschanged to a crystalline phase having low resistance. This result showedthat the Bi₂Te₃ nanowires can be applied as a phase change memorydevice.

In addition, FIG. 12 is a result of examining phase changecharacteristics of the present nanowires by varying its composition,wherein Bi(NO₃)₃5H₂O and TeO₂ are mixed in a ratio of a) 0.01M:0.01M(Bi₂Te₃), b) 0.01M:0.02M(Bi₃Te), c) 0.02M:0.01M (Bi₅Te₃), and d)0.01M:0.005M (Bi₄Te₃).

As shown in FIG. 12, a phase change characteristic, the composition ofwhich bismuth and tellurium is 2:3, was represented (FIG. 12 a).

The bismuth-tellurium nanowires of the present invention have a PRAMcharacteristic, so that it may be used for a phase change memory device.

The invention claimed is:
 1. A phase change memory device comprising asubstrate; and at least one Bi₂Te₃ nanowire formed on said substrate,wherein the nanowire has a PRAM (Phase change Random Access Memory)characteristic representing an amorphous phase (AP) on applying voltagepulses of 7 to 8 V and 30 to 50 ns and a crystalline phase (CP) onapplying voltage pulses of 1 to 2 V and 90 to 110 ns.
 2. The phasechange memory device according to claim 1, wherein it comprises one pairof electrodes applying voltage to said nanowires.
 3. The phase changememory device according to claim 2, wherein said electrode is a metalelectrode.
 4. The phase change memory device according to claim 1,wherein said nanowire is formed in a pore of a template positioned on asubstrate.
 5. The phase change memory device according to claim 4,wherein said template is anodic aluminum oxide (AAO).
 6. The phasechange memory device according to claim 1, wherein the nanowire has adiameter of 10 to 200 nm.
 7. The phase change memory device according toclaim 1, wherein the substrate is silicon, a flexible polyimide film ora polyester film.
 8. A method of preparing a phase change memory deviceof claim 1 comprising steps of immersing a template formed on asubstrate and having at least one pore in an electrolyte containing abismuth precursor and a tellurium precursor; and electrolyticallydepositing Bi_(x)Te_(y), (x/y<1) nanowires in the pores of said templateby applied voltage.
 9. The method of preparing a phase change memorydevice according to claim 8, wherein the template is anodic aluminumoxide (AAO).
 10. The method of preparing a phase change memory deviceaccording to claim 9, wherein the anodic aluminum oxide template has apore size of 10 to 200 nm and a thickness of 50 nm to 800 nm.
 11. Themethod of preparing a phase change memory device according to claim 8,wherein the bismuth precursor is one or more selected from the groupconsisting of Bi(NO₃)₃5H₂O, BiNBO₄, Bi₂VO₅ and BiMe₂(Me₂NCH₂Ph).
 12. Themethod of preparing a phase change memory device according to claim 8,wherein the tellurium precursor is one or more selected from the groupconsisting of TeO₂, CdTe, CdZnTe and HgCdTe.
 13. The method of preparinga phase change memory device according to claim 8, wherein the voltageapplied in the step of electrolytically depositing is a size of −0.5 to1 V.
 14. The method of preparing a phase change memory device accordingto claim 8, further comprising a step of immersing a templateelectrolytically deposited with nanowires in a basic solution todissolve said template.
 15. The method of preparing a phase changememory device according to claim 14, wherein the basic solution is NaOHor KOH.
 16. A method of deleting information in a phase change memorydevice of claim 1 comprising a step of applying voltage pulses of 7 to 8V and 30 to 50 ns to at least one Bi_(x)Te_(y) (x/y<1) nanowire formedon a substrate.
 17. A method of recording information in a phase changememory device of claim 1 comprising a step of applying voltage pulses of1 to 2 V and 90 to 110 ns to at least one Bi_(x)Te_(y) (x/y<1) nanowireformed on a substrate.
 18. An electronic apparatus comprising a phasechange memory device of claim 1.